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Intestinal Absorption Details (Archive)

Notes

Intestinal Absorption Details (Archive)

Sections


Summary

  • Uptake of nutrients and water from the lumen of the digestive tract into the bloodstream and lymphatic vessels.
  • Small intestine (specifically the duodenum) is the major site of nutrient absorption.
  • Transport occurs on both the apical and basolateral surfaces of the intestinal epithelium for absorption into the bloodstream or lymphatic system.
    – Each nutrient utilizes a specific transport mechanism across either surface.

Small Intestine

Monosaccharides -

Proteins and their digested products

  • Free Amino Acids
    – Apical: secondary active transport with Na+
    – Basolateral: facilitated diffusion
  • Di- and Tripeptides
    – Apical: secondary active transport with hydrogen exchange.
    – Basolateral: facilitated diffusion
  • Intact proteins
    – Apical: endocytosis
    – Basolateral: exocytosis

Fats

  • Short-chain fatty acids:
    – Apical and basolateral: simple diffusion
  • Long-chain fatty acids and glycerol:
    – Apical: simple diffusion
  • Long-chain fatty acids and glycerol form chylomicron in small intestine epithelial cell.
    – Basolateral: facilitated diffusion

Vitamins

  • Fat-soluble vitamins (A, D, E, K):
    – Absorbed with fats: dissolved in fat droplets, micelles, and chylomicrons.
  • Water-soluble vitamins:
    – Simple or facilitated diffusion
  • Vitamin B-12: Must be bound to intrinsic factor (IF) for B-12 absorption in the ileum
    – Apical: endocytosis
    – Basolateral: exocytosis

Note: Vitamin B-12 Must be bound to intrinsic factor (IF) for its absorption in the ileum: Intrinsic Factor is secreted in the duodenum where binds with vitamin-B12.

Small and Large Intestine

Water and electrolytes: Remember that water absorption is most relevant to the function of the large intestine for efficient feces concentration prior to elimination.

Three general steps involving water and electrolyte absorption.

  • Step 1
    – Active sodium absorption across the basolateral surface allows facilitated diffusion across the apical surface and creates a net positive charge in the cell
  • Step 2
    – Facilitated chloride diffusion across apical membrane down the electrical gradient.
  • Step 3
    – Water is absorbed via osmosis.

Large Intestine (mainly absorbs water (and also bacterial products))

  • Fiber
    – Converted to short chain fatty acids and absorbed via simple diffusion.
  • Vitamins B and K
    – Dissolved in water for absorption.

Clinical Correlation: Roux-en-Y Procedure

  • Commonly used as a weight-loss (bariatric) surgery: it reduces the stomach size and amount of nutrient absorption.
    – Creates a small gastric pouch that comprises only the fundus and bypasses rest of stomach (the body and antrum) and the duodenum.
    – Jejunum is attached to the gastric fundus.
    – Duodenum attached further down along the jejunum; this allows digestive juices from the stomach and pancreas as well as bile from the gallbladder release into the intestine.
    – "Roux Limb" = portion of the jejunum between two attachments.
  • Following surgery, food passes: esophagus → gastric pouch → Roux Limb → through Roux Limb → remaining jejunum.
    – Because the duodenum is bypassed, these patients lack the absorptive and secretive properties of the duodenum.
    – Patients supplement diets with: iron, vitamin B12, and other vitamins and mineral supplements (no longer absorbed)
    – Also: no longer secrete intrinsic factor from bypassed duodenum → cannot bind to vitamin B-12 in bypassed duodenum → unable to absorb vitamin B12 in ileum.
  • Physicians prescribe vitamin B-12 supplements (nasal spray, sublingual drops, or tablets) → ensures B-12 directly absorbed into circulation.

Full Text

  • In this tutorial, we will track nutrient and water absorption across the small and large intestine epithelium.
    • We will specifically outline each nutrient's key transport mechanisms across both the apical and basolateral surfaces of the intestinal epithelium.
  • First start a table.
  • Begin with the Key Functions of the digestive tract.
  • Denote the following 5 major functions:
    • Motility
    • Secretion
    • Digestion
    • Absorption
    • Protection
    • And the minor function: Elimination.
  • Denote that absorption is the uptake of nutrients and water from the lumen of the digestive tract into the bloodstream and lymphatic vessels.
  • Denote that the small intestine, specifically the duodenum, is the major site of nutrient absorption.

Let's create a table so we can track the absorption of the digested nutrients in the small and large intestine.

  • In the first column, label Nutrients, which are within the chyme in the small intestine lumen.
  • In the next column, label Transport.
  • Subdivide this column into
    • Apical
    • Basolateral
  • Denote transport occurs on both the apical and basolateral surfaces of the intestinal epithelium for absorption into the bloodstream or lymphatic system.
    • Each nutrient utilizes a specific transport mechanism across either surface.
  • Further divide the table into three panels
    • Small intestine
    • Small and large intestine
    • Large intestine
  • Most nutrients are absorbed in the small intestine; however, some are absorbed in both the small and large intestines while others are absorbed specifically in the large intestine.

First, for the small intestine let's address the monosaccharides.

  • For apical transport
    • Glucose and galactose cross via secondary active transport with Na+
    • Fructose crosses via facilitated diffusion

Now, let's address proteins and their digested products.

  • For apical transport
    • Free amino acids cross via secondary active transport with Na+
    • Di- and tripeptides cross via secondary active transport with hydrogen exchange
  • Next, write that monosaccharides, free amino acids, and di- and tripeptides all cross the basolateral surface via facilitated diffusion.
  • Now, indicate that intact proteins are endocytosed for apical transport and exocytosed for basolateral transport.
    • As a clinical correlate, infants absorb maternal antibodies from breast-milk via endocytosis and exocytose the antibodies into the bloodstream.

Now, let's address fats.

  • Indicate that short-chain fatty acids cross the apical and basolateral surfaces via simple diffusion.
  • Next, indicate that glycerol and long-chain fatty acids cross the apical surface via simple diffusion.
    • Once inside the enterocyte, glycerol and long-chain fatty acids form chylomicrons.
    • Indicate that chylomicrons cross the basolateral surface via facilitated diffusion.

Let's now address vitamins.

  • Indicate that fat-soluble vitamins (A, D, E, and K) cross the apical and basolateral surfaces dissolved in fat droplets, micelles [my-cells], and chylomicrons.
  • Indicate that in general, water-soluble vitamins are absorbed by simple or facilitated diffusion via a specific transporter.
    • However, water-soluble vitamin B-12 crosses the apical surface via receptor-mediated endocytosis and basolateral surface via exocytosis.
    • Indicate that vitamin B-12 must be bound to Intrinsic Factor (IF) for its endocytosis in the ileum.
    • Intrinsic Factor is secreted in the duodenum where binds with vitamin-B12.

Let's now transition to the small and large intestine.

  • Indicate that water and electrolyte absorption occurs in both the small and large intestine. Remember that Water absorption is most relevant to the function of the large intestine for efficient feces concentration prior to elimination.
  • Write the three general steps involving water and electrolyte absorption.
  • Step 1: There is active sodium absorption across the basolateral surface, which allows facilitated diffusion across the apical surface.This step creates a net positive charge in the cell, which sets up step 2.
  • Step 2: There is facilitated chloride diffusion across the apical membrane down the electrical gradient.
  • Step 3: Water is absorbed via osmosis.

Lastly, let's transition to the large intestine.

  • While it primarily absorbs water, write that it also absorbs a variety of bacterial products, such as:
    • Fiber, which is converted to short chain fatty acids and absorbed via simple diffusion.
    • Vitamins B and K, which are dissolved in water for absorption.
  • As a clinical correlation, let's draw a standard Roux-en-Y.
  • Denote that the Roux-en-Y procedure is commonly now used as a weight-loss (bariatric) surgery; it reduces the stomach size and amount of nutrient absorption.
  • Draw the lower portion of the esophagus extending into a small gastric pouch that comprises only the fundus.
  • Draw the bypassed stomach (the body and antrum) and the bypassed portion of the small intestine (the duodenum).
  • Next, draw the jejunum and attach it to the gastric fundus.
  • Now, attach the duodenum further down along the jejunum, which allows digestive juices from the stomach and pancreas as well as bile from the gallbladder release into the intestine.
  • Indicate the "Roux Limb" as the portion of the jejunum between these two attachments.
  • Use arrows to show the passage of food (following surgery) from the esophagus into the gastric pouch, through Roux Limb, and into the rest of the jejunum.
    • Because the duodenum is bypassed, these patients lack the absorptive and secretive properties of the duodenum.
    • Therefore, these patients supplement their diets with iron, vitamin B12, and other vitamins and mineral supplements that are no longer absorbed.
  • Also, indicate that because these patient's no longer secrete intrinsic factor from the bypassed duodenum, it cannot bind to vitamin B-12 in the bypassed duodenum.
    • Therefore, they are unable to absorb vitamin B12 in the ileum.
    • Physicians prescribe vitamin B-12 supplements in the form of nasal spray, sublingual drops, or tablets to ensure the vitamin is absorbed directly into circulation as there is no longer a system to utilize B-12 if taken orally.